A trailer adapted at the rear of a bicycle for towing thereby is primarily provided to carry small children and further to interact with them. Since such a trailer has a certain volume and is used outdoors, the frame of the trailer is usually foldable for convenient transportation and storage. A conventional frame is in U.S. patents, such as U.S. Pat. Nos. 5,947,497, 5,474,316 and 5,577,746. In general, the frame includes a lower frame member and an upper frame member having a rear end pivotally connected to a forward end of the lower frame member and extending upward; and a pair of pivot units, each of which has one end pivotally connected to the lower frame member and the opposite end pivotally connected to the upper frame member, defining a cargo compartment. The frame of the trailer is extended by means of the pair of pivot units positioned in a line or in parallel that is fixed by a fastened member, so as to support the upper frame member in an extended position. The frame of the trailer is collapsed by loosening the fastened member to make the pair of the pivot units rotate towards the frame, allowing a pivot point between the upper frame member and the lower frame member to rotate downward and thereby fold the trailer into a compact position.
The aforementioned trailer is shifted inwards to the frame and is placed inside thereof after the pair of the pivot members are folded. Hence, it is quiet inconvenient to extend the folded frame.